The object of this proposal is to study the molecular action of ionizing radiation and ultraviolet radiation on DNA. Radiation effects on both the single-strand mature form of bacteriophage phiX174 DNA and its circular covently closed double-strand replicative intermediate (called "RFI") will be studied. Purified DNA in dilute aqueous solution will be exposed to ionizing radiation (ior) and ultraviolet (UV) radiation. Physical damage will be measured by the appearance of strand breaks. Biological damage will be measured as the loss of ability of the DNA to form phage when the DNA is incubated with calcium-treated cells. The radiosensitivities of the single-strand and the double-strand forms of the DNA will be compared. Also, the ability of cellular repair systems, in particular the inducible repair pathway, to enhance survival for ior and UV irradiated single-strand and double-strand DNA will be studied. Bacterial cells are known to be sensitized to radiation killing by incubation with the antiobiotic rifampicin prior to irradiation. In vitro studies have shown that rifampicin inhibits RNA synthesis by inhibiting the initiation of RNA synthesis. RNA polymerase binds to DNA in the presence of rifampicin, but it is not able to begin the synthesis of the RNA polymer. The possible influence of RNA polymerase association with the phiX174 molecule on its radiosensitivity will be studied. These in vitro experiments should help to determine whether RNA polymerase association with cellular DNA at the time of irradiation influences the radiosensitivity of cells. The changes in radiosensitivity of bacterial cells as a function of their physiological state (for example, log phase vs. stationary phase) and mammalian cells as a function of their cell cycle stage could in part be related to variations in RNA polymerase association with the DNA.